The organic analyses of environmental samples involve the separation of analytes (components) of interest from such matrices as soil, water, fly ash, tissue or other material. Liquid extraction is traditionally used as the separation process. For example, water samples are usually extracted with organic solvent. Similarly, solid samples are leached with an organic solvent in a SOXHLET apparatus. Methods based on solvent extraction are often time consuming, difficult to automate and are very expensive since they require high purity organic solvents and these organic solvents involve significant purchase and disposal costs. Further, the organic samples may have high toxicity and often are difficult to work with. In addition, the extraction processes can be highly nonselective. Therefore, sequential chromatographic techniques must sometimes be used to separate complex mixtures after extraction, significantly increasing the overall analysis time and the cost.
Solid phase extraction is a known effective alternative to-liquid-liquid extraction in the analysis of aqueous samples. The primary advantage of solid phase extraction is the reduced consumption of high purity solvents and the resulting reduction in laboratory costs and solvent disposal costs. Solid phase extraction also reduces the time required to isolate the analytes of interest. However, solid phase extraction continues to use solvents and often suffers from high blank values. Further, there is considerable variation between the products offered by different manufacturers and lot-to-lot variation can be a problem when carrying-out solid phase extraction procedures. Solid phase extraction-cartridges available from manufacturers are typically constructed of plastic, which can adsorb the analytes and increase interferences in the analysis. The disposable plastic cartridges used in the solid phase extraction process are first activated using organic solvent. The excess organic solvent is then removed and the sample to be tested is passed through the cartridge. The organic analytes from the sample are adsorbed on the chemically modified silica surface of the material in the cartridge. Both molecules of interest as well as interferences are retained on the cartridge material. During desorption, a selective solvent is chosen to first remove the interferences. The analyte is then washed out of the cartridge. The analytical procedure from that point on is identical to that used in liquid-liquid extraction. The analyte is first pre-concentrated by evaporating down the extract and the mixture is then injected into an appropriate high resolution chromatographic instrument. Steps involving the use of organic solvents are the most time consuming.
Solid phase microextraction, or SPME, was developed as the alternative to the foregoing prior art methods of preparing samples in a fluid carrier for chromatographic analysis; see Pawliszyn, Janusz, WO 91/15745, International Publication Date of Oct. 17, 1991. SPME involves using a fiber that is mounted within a hollow needle of a syringe, e.g. a modified gas chromatography (GC) syringe. The fiber, for example a fused silica fiber coated with an adsorbent or a stationary phase, acts as a "sponge" to extract a sample and to concentrate the organic analytes on its surface so that it can be transferred into the heated injector of the GC. While in the injector, the analytes are thermally desorbed from the fiber and transferred into the GC column for analysis. With SPME, one can achieve detection limits down to the parts-per-trillion (ppt) range for a wide number of volatile and semi-volatile compounds. Pertinent portions of the Pawliszyn reference that define details of the SPME unit are incorporated by reference herein.
The chief disadvantage of the use of SPME is the time required to extract the sample by the coated fibers. For example, when a water matrix sample containing one or more analytes of interest is desired to be analyzed and is contained in a typical sample vial containing a septum, the needle of the syringe of the SPME device is inserted through the septum. The plunger of the syringe is depressed and the exposed coated fiber extends from the free end of the needle and is inserted either above (headspace sample) or into the water matrix sample (liquid sample). In this manner, the fiber will not be damaged by the septum of the sample vial. For example, organic analytes that may be found in water can be extracted into a non-polar phase coated onto the fiber. Water is considered to be the carrier in a water matrix sample. When the microextraction has occurred to a sufficient degree, the plunger is moved to the withdrawn position causing the fiber to be drawn into the needle and the needle is removed from the sample bottle through the septum. The time for fiber adsorption of the analytes to be extracted will depend on many factors including the analytes themselves as well as the thickness and type of coating, if any, on the fiber. Typically the equilibrium adsorption time ranges from 1 to 30 minutes, with some analytes requiring up to several hours. In the preferred method of operating SPME, the sample is stirred or the vial is rotated to impart forceful agitation of the sample during the time the fiber is present in the vial during the extraction stage of the analysis in order to decrease the adsorption time. The stirring can be done by placing a magnetic bar within the analyte and by using a conventional magnetic stirrer. Another method for agitation is to induce ultrasonic vibrations within the liquid sample in the vial. It has been found that the adsorption time can be reduced from about 30 minutes range to approximately two minutes with forceful agitation; see FIG. 9 at page 1194 of D. Louch, S. Motlagh, and J. Pawliszyn, "Dynamics of Organic Compound Extraction From Water Using Liquid-Coated Fused Silica Fibers", Analytical Chemistry, Vol. 84, No. 10, pages 1187-1199 (May 15, 1992).
After the extraction stage, the plunger is moved to a withdrawn position to retract the fiber within the needle and the needle is removed from the bottle. During the analysis stage, the needle is inserted through the septum of an injection port of a conventional gas chromatograph or other suitable analytical instrument and the analytes are then desorbed into the injector port.
It has been found that to provide sufficient sample agitation to significantly reduce the adsorption time using the above method, mechanical and electrical part damage can occur. Under some cases of forceful agitation, the vials have been known to crack and even to break. In addition, the use of magnetic, ultrasound and other conventional stirring means added to the sample introduces a potential source of contamination. A disadvantage of using ultrasound agitation of the sample is the unwanted rise in the temperature of the sample which adds an unwanted and uncontrollable variable to the analysis since adsorption efficiency is temperature dependant. Another disadvantage of the prior art SPME technique is the slow rate of absorption as a result of the coating thickness of the stationary phase on the fibers. The coating thickness is dictated by the capacity of the stationary phase to absorb the analytes.
Murphy, U.S. Pat. No. 5,565,622, discloses a method for overcoming many of the problems of the previous SPME method by microextraction onto the inner surface of a syringe needle at least partially coated with the stationary phase followed by desorption of the absorbed components into the gas chromatograph injector, either thermally or using a solvent flush. If the components are thermally desorbed from the inner surface of the needle, there is an inefficient transfer into the chromatographic column. The first problem with thermal desorption is that as the needle heats up, the absorbed components vaporize, but are not swept into the injector by any directing force. Secondly, since the needle is inserted into the pressurized zone of the injector, the pressurized carrier gas tends to reverse flow through the needle causing loss of sample. This is the case because the syringe plunger/barrel assembly commonly have leaks. On the other hand if a solvent flush is used, one of the chief advantages of using the SPME method is negated. This is true because the peak of the chromatogram for the solvent interferes with the peaks for the components under analysis. Finally, Murphy also discloses using cryotrapping of the components of interest on the head of the chromatographic column prior to analysis. Cryotrapping is a known method for increasing column efficiency, i.e., obtaining good component peak shapes. The problem with using cryotrapping is that the amount of analytes present in a given sample vial are the maximum that can be absorbed onto the stationary phase by the Murphy method.
There is a need for an alternative method to improve the rate of desorption without the necessity of using either thermal desorption or a solvent flush of the prior art methods. There is also a need to increase the amount of analytes that can be absorbed onto the coated needle than with the cryotrapping methods of the prior art.